Here you have some primary sources from Sahih Bukhari, book 65.:
Volume 7, Book 65, Number 305:
Narrated Nafi':
Ibn 'Umar never used to take his meal unless a poor man was called to eat with him. One day I brought a poor man to eat with him, the man ate too much, whereupon Ibn 'Umar said, "O Nafi'! Don't let this man enter my house, for I heard the Prophet saying, "A believer eats in one intestine (is satisfied with a little food), and a kafir (unbeliever) eats in seven intestines (eats much food)."
Volume 7, Book 65, Number 306:
Narrated Ibn 'Umar:
Allah's Apostle said, "A believer eats in one intestine (is satisfied with a little food), and a kafir (unbeliever) or a hypocrite eats in seven intestines (eats too much)."
Volume 7, Book 65, Number 307:
Narrated 'Amr:
Abu Nahik was avaricious eater. Ibn 'Umar said to him, "Allah's Apostle said, "A Kafir (unbeliever) eats in seven intestines (eats much)." On that Abu Nahik said, "But I believe in Allah and His Apostle ."
Volume 7, Book 65, Number 308:
Narrated Abu Huraira:
Allah's Apostle said, "A Muslim eats in one intestine (i.e. he is satisfied with a little food) while a Kafir (unbeliever) eats in seven intestines (eats much)."
Volume 7, Book 65, Number 309:
Narrated Abu Huraira:
A man used to eat much, but when he embraced Islam, he started eating less. That was mentioned to the Prophet who then said, "A believer eats in one intestine (is satisfied with a little food) and a Kafir eats in seven intestines (eats much). "